I got mine done for £500 including VAT and fitting etc... Longlife Carshalton... Pretty decent chap did it but I think if I have another exhaust then I will go with a recommended manufacturer like eisenmann or similar....
I've changed the back boxes and induction system all in the pursuit of more noise and I drive to the Côte d'Azur every year. Not all increases in noise lead to drone and uncomfortable long distance driving